I have tried cutting the json and creating an object in the console and it works well. I also change the url in order to fetch data from twitter api and also work. Seems that the problem is about pipes.

Thank you very much mitchell. With your help I solved the problem. The thing is that by specifying that my store uses jsonp, Sencha automatically added a callback to my url as I was able to see through Chrome. So it looks like http://pipes.yahoo.com/pipes/pipe.run?...&callback=Ext.data.JsonP.callback1

But pipes expects its jsonp call to have as name _callback(with an uderscore before the name) as specified here http://pipes.yahoo.com/pipes/docs?do...leshooting#q12 so I added the parameter callbackKey: '_callback' to the proxy definition and all worked ok!!!! Thank you very much again